Electrolytic apparatus and process

ABSTRACT

An improved electrolytic apparatus and process particularly applicable for high-speed electrodeposition of a metallic plating on a substrate such as on a printed wiring board of the type employed in the electronic industry. The apparatus employs a manifold assembly adapted to be immersed in the electrolyte and positioned adjacent to each of the anodes immersed therein for withdrawing electrolyte past the anode to provide a controlled rate of flow across the anode surface to achieve a substantially uniform mass transport rate across the fluid film on the surface of the anode. The apparatus is also adapted for returning at least a portion of the withdrawn electrolyte to the tank which can be arranged so as to impinge upon the article being plated.

This is a division of co-pending application Ser. No. 825,112, filed onFeb. 3, 1986, now U.S. Pat No. 4,687,554.

The present invention broadly relates to an improved electrolyticapparatus and process, and more particularly, to an improved high-speedelectroplating cell and process for effecting a rapid electrodepositionof a metallic plating on a substrate. More specifically, the presentinvention is directed to an improved electrolytic cell and process forelectrodepositing copper on a substrate, and particularly, on printedwiring boards of the types employed in the electronic industry.

The art of electroplating is well known and in widespread use inindustry for electrodepositing a variety of metals and metal alloys onsubstrates to provide a functional or decorative electrodeposit on thesubstrate or selected portions thereof. The adoption of printed wiringboards in the electronic industry has prompted a need for electrolyticapparatuses and processes for effecting a more rapid deposition of metaldeposits, particularly copper on the wiring board providing for auniform, adherent electrodeposit which is of the desired thickness andwhich also covers the interior surfaces of through-holes in the wiringboard to which electronic components are subsequently wired. Attempts toincrease the plating rate of such electrolytic devices of the typesheretofore known has not been successful due to the tendency of formingnon-uniform electrodeposits, a so called "burning" of the deposit inlocalized areas thereof, a passivation of the soluble anodes employed inthe electrolyte and/or a nonuniform coverage of the aperture or holes inthe wiring board. Attempts to eliminate one or more of these problems byproviding air agitation and other mechanical agitation techniques havenot been entirely successful.

One such prior art electrolytic apparatus is described in U.S. Pat. No.4,174,261 granted on Nov. 13, 1979 and entitled "Apparatus forElectroplating, Depleting or Etching". According to the apparatusdescribed in the aforementioned U.S. Patent, a plurality of spraynozzles or jets are provided which are adapted to discharge theelectrolyte directly against the surface of the substrate, such as aprinted wiring board, to be plated while a suction port is providedadjacent to the spray nozzle for immediately withdrawing the electrolytefrom the vicinity of the article being plated. While an improvement inthe rate of deposition of the plating metal is achieved, nevertheless,the rate of electrodeposition is less than optimum.

In accordance with the electrolytic device and process of the presentinvention, a substantial improvement is achieved in the rate ofelectrodeposition of a metal such as copper or an article such as aprinted wiring board whereby a substantial improvement in productioncapacity can be achieved, while at the same time providing for uniform,adherent electrodeposits over all or the selected areas to be plated.The apparatus is further characterized as being adaptable forinstallation in conventional prior art-type electrolytic apparatusesenabling their operation in accordance with the present invention andachieving the improved benefits and economies resulting therefrom. Theelectrolytic apparatus and process of the present invention is furthercharacterized by its simplicity in construction, ease of operation andcontrol, and efficiency and durability in operation.

SUMMARY OF THE INVENTION

The benefits and advantages of the present invention are achieved by anelectrolytic apparatus including a tank or receptacle adapted to containa body of electrolyte therein. Supporting means are provided forsupporting a workpiece to be electroplated in immersed relationship inthe electrolyte. The supporting means may conventionally comprise anelectrified cathode bar for cathodically electrifying the workpieceduring an electroplating operation. The apparatus further comprises ananode support for supporting at least one, preferably a plurality ofsoluble anodes in immersed relationship in the electrolyte in spacedrelationship from the workpiece. The anode support preferably comprisesan electrified anode bar for anodically electrifying the anodes duringan electroplating operation in a manner to cause electric current toflow between the workpiece and the anode. A manifold assembly is furtherprovided which is immersed in the electrolyte and positioned adjacent toat least one side of each of the anodes therein including a plurality ofinlet ports for withdrawing electrolyte at a controlled flow rate acrossthe surfaces of the anodes to achieve a substantially uniform masstransport rate across the anode fluid film to avoid anode passivationand to simultaneously preserve the anode film. This can be achieved bycontrolling the velocity of the electrolyte at the surface of the anodewithin a range of about 0.3 to about 6 feet per second, preferably, fromabout 1 to about 3 feet per second.

In accordance with a preferred embodiment of the present invention, theelectrolyte withdrawn through the manifold assemblies is returned to theelectroplating tank and can be discharged in the form of jets disposedin close proximity to the surface of the article assuring uniformagitation over the surfaces to be plated. The agitation of theelectrolyte in the vicinity adjacent to the anode surfaces occasioned bythe controlled withdrawal of electrolyte through the manifold assemblydispenses with the need of employing supplemental agitation devices suchas air agitation, for example.

The elimination of air agitation in accordance with the presentinvention overcomes the problems associated in prior art plating cellsemploying air agitation as a result of variations in air flow resultingin non-uniform agitation in the vicinity of the cathode and anodes whichin some instances results in destruction of the anode film and theformation of a non-uniform electrodeposit on the cathode being plated.In addition, the use of air agitation can cause irregularities and/ornon-uniform local current distribution as well as increased voltagerequirements due to the increased ohmic resistance of air voids in theelectrolyte. The elimination of air agitation overcomes the foregoingproblems and provides the further benefit in a reduction in theoxidation rate of the organic additive components in the electrolytethereby achieving better process control and a reduction in theconsumption of such organic additives. Additionally, the elimination ofair and its tendency to oxidize such additives further reduces theconcentration of contaminating by-products in the electrolyte. Theelimination of the need for air agitation further reduces the formationof mist above the plating cell providing an improved working environmentand reducing ventilation and scrubbing of mist in the exhaust gases.

In addition to the foregoing advantages, the elimination of airagitation further provides for a substantial reduction in the tendencyto cause foam formation on the surface of the electrolyte. Additionally,the elimination of air agitation further eliminates a source ofcontamination of the electrolyte by oils and lubricants entrained in theair employed for air agitation with a corresponding elimination orreduction in the need for air purification equipment.

In accordance with the process aspects of the present invention, thepreservation of high-plating efficiency is maintained by the controlledwithdrawal of electrolyte from a vicinity adjacent to the soluble anodesimmersed in the electrolyte in a manner to provide a substantiallyuniform mass transport rate across the fluid film on the surfaces of theanode thereby preventing anode passivation and avoiding destruction ofthe anode film thereon. Optimum replenishment of the metal ions in theelectrolyte is achieved by the progressive dissolution of the anodesproviding for electrodeposition rates heretofore unobtainable inaccordance with prior art apparatuses.

Additional benefits and advantages of the present invention will becomeapparent upon a reading of the description of the preferred embodimentstaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view, partly in section and partlyin phantom, of an electrolytic apparatus constructed in accordance withthe preferred embodiments of the present invention;

FIG. 2 is a transverse vertical sectional view of the apparatus shown inFIG. 1; and

FIG. 3 is a longitudinal vertical sectional view of the apparatus shownin FIG. 2 and taken along the line 3--3 thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the drawings, an electrolytic apparatusconstructed in accordance with the preferred embodiments of the presentinvention comprises a tank or receptacle 10 adapted to contain anelectrolyte of the desired composition such as a copper electrolyte. Thetank 10 is constructed of a suitable corrosion resistant material suchas polyvinyl chloride or may comprise a corrosion resistant metal suchas stainless steel having a protective plastic coating, such asplastisol coating thereover. As best shown in FIGS. 2 and 3, the tank 10is supported on a suitable box framework indicated at 12 including basebeams 14, upright beams 16 and longitudinally extending side beams 18.

A cathode bar 20 is suitably supported and extends longitudinally abovethe center line of the tank 10 from which a workpiece 22, such as aprinted circuit board, is suspended by means of a pair of supportinghooks 24. In accordance with conventional practice, the cathode bar 20is connected to an electric DC source for effecting a cathodicelectrification thereof which is transferred through the conductivesupporting hooks 24 to the workpiece 22 disposed in electricalconnection therewith.

As may be best seen in FIGS. 1 and 2, a pair of anode bars 26 aresupported along each side of the tank 10 in spaced relationship fromeach side of the workpiece 22 and from which a plurality of solubleanodes 28, such as copper anodes, are suspended from hooks 30 inelectrical contact with the anode bar. The anode bars 26 are similarlyelectrically connected to a DC power source for effecting anodicelectrification of the anode bars and the anodes suspended therefrom.The electrolyte solution level is controlled at an appropriate height inthe tank so as to maintain the anodes immersed therein such as at alevel indicated at 32 in FIG. 2.

As shown in the drawings, a perforated manifold 34 is disposed in asubstantially upright position adjacent to and inwardly of each anode 28and is formed with a plurality of inlet ports 36 disposed at spacedintervals along the length thereof. The location, number and geometry ofthe suction manifolds 34 and the number of the inlet ports 36 therealongprovides for more uniform electrolyte flow conditions in the tank orplating cell in comparison to the use of a localized return pipe orpipes in accordance with prior art practices. The more uniformelectrolyte flow in the tank or plating cell simultaneously provides formore uniform flow and replenishment of electrolyte in the vicinity ofthe cathode being plated. The lower end of each of the perforatedmanifolds 34 are connected by means of a coupling 38 to a suction headerconduit 40 which extends longitudinally along the bottom of the tank inupwardly spaced relationship therefrom. The right hand end of thesuction header 40 as schematically illustrated in FIG. 3 of the drawingis connected by means of an upright conduit 42 which passes through theside wall of the tank 10 and in turn is connected to a conduit 44provided with a valve 46 which is connected to the suction or inlet sideof a pump 48. The flow control valve 46 is adapted to be adjusted so asto control the volume of electrolyte withdrawn through the inlet portsof the manifolds 34 to achieve an appropriate rate of flow ofelectrolyte across the surfaces of the anodes 30 to achieve asubstantially uniform mass transport rate while at the same time,avoiding the destruction of the anode film. A substantially uniform masstransport rate without destruction of the anode film can be achieved bycontrolling the velocity of the electrolyte across the anode surfacewithin a range of about 0.3 to about 6 feet per second, and preferably,from about 1 to about 3 feet per second. The particular velocity atwhich optimum efficiency is attained will vary depending on theparticular anode geometry and spacing.

In accordance with a preferred embodiment of the electrolytic apparatus,the electrolyte withdrawn through the perforated manifolds and suctionheader 40 is recirculated through a conduit 50 connected to the pressureside of the pump 48 which is connected to an upright conduit 52 thatextends downwardly along the inner side wall of the tank 10 as shown inFIG. 3. The conduit 52 is connected to a pressure header 54 extendinglongitudinally along and spaced from the base of the tank at a positioninwardly of the suction headers 40 and adjacent to workpiece 22 immersedtherein. As best seen FIGS. 1 and 2, a plurality of perforated dischargemanifolds 56 are connected by means of couplings 58 at their lower endsto the pressure header 54 and extend upwardly in spaced substantiallyparallel relationship adjacent to the side surfaces of the workpiece 22.The discharge manifolds 56 are provided with a plurality of dischargeports or nozzles 60 oriented in the direction of the side surfaces ofthe workpiece 22 to discharge electrolyte against the surfaces effectinga continued supply of replenished electrolyte.

The suction manifolds 34 and discharge manifolds 56 are retained inappropriate position by means of a header plate 62 affixed to andextending across the upper ends thereof. A spacer plate 64 is disposedbelow the suction header 40 and pressure header 54 and secured theretofor maintaining the headers in appropriate spaced parallel relationshipand spaced upwardly from the surface of the bottom of the tank. The sizeand number of discharge nozzles in the discharge manifolds 56 isselected and controlled so as to provide for substantially uniformcoverage of each side surface of the workpiece 22 with fresh electrolyteduring an electroplating operation.

It is also contemplated, that in lieu of the discharge manifolds 56, allor a portion of the electrolyte withdrawn through the suction manifolds34 can be directly discharged into the plating tank, preferably in amanner to provide for solution agitation at a position adjacent to theworkpiece 22.

In accordance with the specific arrangement shown employing thedischarge manifolds 56, optimum electroplating of the workpiece isachieved by adjusting the flow control valve 46 to provide forcontrolled electrolyte withdrawal from a vicinity adjacent to theanodes. Similarly, in such instances in which the quantity ofelectrolyte withdrawn through the suction manifolds 34 is insufficientto provide for optimum electrolyte discharge from the dischargemanifolds 56, a second flow control valve 66 is provided as best seen inFIG. 3 which can be opened to withdraw supplemental electrolyte througha conduit 68 disposed adjacent to the base of the tank to provide theappropriate total electrolyte volume discharged through the dischargemanifolds. In order to provide better control of the total volume ofelectrolyte discharged from the discharge manifolds, a flow controlvalve 82 is incorporated in the discharge conduit 50 which can beadjusted for total return volume while retaining valves 46 and 66 in apreset adjusted position.

In accordance with a further preferred embodiment of the presentinvention and as best shown in FIGS. 1 and 2, a pair of current shields70 are secured in an upright position and extend longitudinally alongthe center line of the tank at a position spaced from the lower portionof the workpiece 22. The use of current shields provides for a betterdistribution of the electrical current flowing between the anode andworkpiece thereby achieving a more uniform thickness of electrodepositover the entire area or selected sections of the workpiece 22.

In operation, a workpiece 22 to be electroplated is automatically ormanually transferred to a position in suspended relationship from thecathode bar 20 at a position such as shown in FIG. 2. The pump 48 (FIG.3) is energized and the flow control valves 46, 66, and 82 are adjustedto provide for optimum electrolyte withdrawal and discharge during theelectroplating operation. In accordance with conventional practice, therecirculated electrolyte can be subjected to filtration to removeextraneous particulate matter therefrom and can also be replenished withadditional chemical agents to maintain the composition thereof withinappropriate limits.

In order to further illustrate the benefits of the present invention,tests were conducted in a laboratory prototype plating cell having thegeneral arrangement as shown in FIG. 1 of the drawings and having acapacity of 100 gallons (about 380 liters) electrolyte. The tank wasfilled with an acidic copper electrolyte of the type typically employedfor plating printed wiring boards nominally containing about 27 gramsper liter (g/l) copper ions, 0.07 g/l chloride ions, 262 g/l sulfuricacid and minor amounts of conventional proprietory organic brighteningadditives.

A printed wiring board was immersed in the electrolyte and phosphorizedbar copper anodes of a standard type were immersed and anodicallycharged. The electrolyte was controlled at a nominal temperature ofabout 40° C.

A series of tests were conducted in which the flow rate expressed interms of gallons per minute of electrolyte was withdrawn from the tankthrough the suction manifolds. The quantity was varied in incrementalsteps and the anode current density was progressively increased todetermine the maximum satisfactory operating anode current density foreach flow rate as well as the current density at which anode passivationoccurred. The results are tabulated in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Anode Current Density, ASF                                                                    Current Density at                                  Suction Manifold                                                                          Max. Satisfactory                                                                           Which Anode                                         Flow Rate, gpm                                                                            Current Density                                                                             Passivation Occurred                                ______________________________________                                        0           110           115                                                 1           115           120                                                 2           120           125                                                 4           130           135                                                 8           180           200                                                 16          200           225                                                 ______________________________________                                    

As will be noted in Table 1, a substantial increase in the anode currentdensity was possible for providing satisfactory plating operation as theflow rate of withdrawn electrolyte through the suction manifoldsincreased. The magnitude of anode current density is indicative ofplating rate indicating that a substantial increase in the rate ofcopper deposition can be obtained in accordance with the presentinvention.

The foregoing tests were repeated employing an electrolyte temperaturecontrolled at about 50° C. and similar results were obtained indicatinga negligible effect of electrolyte temperature on the results.

While it will be apparent that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope or fair meaning of the subjoined claims.

What is claimed is:
 1. An electrolytic apparatus comprising;(a) a tankadapted to contain a body of an electrolyte, (b) work support means forsupporting a workpiece to be processed in immersed relationship in saidelectrolyte, (c) means for cathodically electrifying the workpiece, (d)anode support means for supporting at least one soluble anode inimmersed relationship in said electrolyte in spaced relationship fromthe workpiece, (e) means for anodically electrifying said anode to causeelectric current to flow through the electrolyte between said anode andthe workpiece, (f) a manifold assembly immersed in the electrolyte andpositioned adjacent to at least one side of said anode and including atleast one inlet port, and (g) pump means connected to said manifoldassembly for withdrawing electrolyte from said tank past the anode toprovide a controlled rate of flow of electrolyte across the anodesurface and into the inlet port thereof to achieve a substantiallyuniform mass transport rate across the fluid film on the surface of theanode.
 2. The apparatus as described in claim 1 further including returnmeans for returning at least a portion of the withdrawn electrolyte tosaid tank.
 3. The apparatus as described in claim 1 further including aplurality of discharge manifolds for discharging electrolyte underpressure against the surface of a workpiece to be processed.
 4. Theapparatus as described in claim 3 further including conduit meansconnecting said pump means to said discharge manifolds.
 5. The apparatusas described in claim 1 further including flow control means forcontrolling the rate at which electrolyte is withdrawn through saidmanifold assembly by said pump means.
 6. The apparatus as described inclaim 3 further including second flow control means for controlling therate at which electrolyte is discharged from said discharge manifolds.7. The apparatus as described in claim 1 including a plurality of anodesdisposed in relative spaced relationship and said manifold assemblycomprises a plurality of suction manifolds disposed in relative spacedrelationship and adjacent to said anodes.
 8. The apparatus as describedin claim 1 including a plurality of anodes disposed in relative spacedrelationship along each side of a workpiece to be processed and saidmanifold assembly comprises a plurality of suction manifolds disposed inrelative spaced relationship and adjacent to said anodes.
 9. Theapparatus as described in claim 1 in which said manifold assemblycomprises a plurality of elongated suction manifolds each formed with aplurality of inlet ports disposed in spaced relationship therealong. 10.The apparatus as described in claim 1 in which said manifold assemblycomprises a header connected to said pump means and a plurality ofelongated suction manifolds connected in communication with said header,each of said suction manifolds formed with a plurality of inlet portsdisposed at spaced intervals therealong.
 11. The apparatus as describedin claim 10 in which said suction manifolds are disposed in asubstantially upright position in spaced relationship along said header.12. The apparatus as described in claim 1 in which said manifoldassembly includes at least one elongated suction manifold formed with aplurality of inlet-ports at spaced intervals therealong.
 13. Theapparatus as described in claim 12 in which suction manifold and saidinlet ports therealong extend for substantially the length of the anodeadjacent thereto.
 14. The apparatus as described in claim 12 in whichsaid suction manifold is disposed at a position between said anode andthe workpiece.
 15. The apparatus as described in claim 3 in which eachof said discharge manifolds is of an elongated configuration and isformed with a plurality of discharge nozzles disposed at spacedintervals therealong.